Implications of water column ammonium uptake and regeneration for the nitrogen budget in temperate, eutrophic Missisquoi Bay, Lake Champlain (Canada/USA)

Abstract

Assessing ammonium (NH4 +) availability in aquatic systems requires accurate concentration and turnover rate data. Water column NH4 + regeneration, potential NH4 + uptake, and nitrogen (N) fixation rates were measured in Missisquoi Bay, Lake Champlain, to help constrain internal N dynamics affecting phytoplankton community structure and cyanobacteria (Cy) blooms. Cyanobacteria dominated phytoplankton biomass during occasional summer bloom periods, but low or undetectable N2 fixation rates and low heterocyte abundances suggested that N fixing cyanobacteria did not rely on atmospheric N2. Light/dark incubations revealed that photosynthetic and dark NH4 + uptake generally were balanced, highlighting the importance of bacterial uptake. Our results suggest that phytoplankton were not controlled by nutrients from the “bottom-up”; rather, water column N dynamics responded to phytoplankton patterns. Basin-scale water column NH4 + regeneration rates were about 700,000 mol N day−1 (9.8 t N day−1), which is almost twice the estimated N load from tributaries, and suggests a primary role for water column N regeneration in supporting primary production. Comparisons of basin-scale NH4 + regeneration and demand imply that primary production is not sustained fully by combined water column regeneration and tributary N inputs; thus, future research should constrain additional sources and sinks affecting N balance in this and other aquatic systems.